First the short answer, and then the explanation which will lead to more kaizo if you are interested!

Short answer:

After beating any%, you would need to figure out a puzzle to proceed.
However, if you don't feel like figuring that out, here is the solution:

In the first room of the level "Defiant Jazz", enter the passcode "B, Down, B, Left, Right, B, B, Down, B, Left, Right, B"
In the next room, run to number 4, then count 28 tiles and jump up to enter the invisible door.

If you would like to see what the puzzle leads to, search for InalienableDreamless in the SMW Central member list!



Long answer/explanation:

After you complete any% (reaching the credits in Tremond), you will need to figure out a puzzle to proceed.  When you go up from Tremond, 
there is another level named Defiant Jazz which has an info box that says "Only the bookhouse boys can face the evil beyond."

There is a one-level screen after two of the switch palaces - Grasshopper and Speed With Teeth.  Each of these have info boxes.

If you move up on the OW from the level Rave On, there is a level named The Missing Pieces.  If you jump over the orb, you 
will hear a checkpoint.  Then run into the spike.  When respawning, you will be thrown towards the door.  Go in that door and there will
be info boxes with the corresponding missing letters.

Overlaying, or typing out every other letter will lead to the following messages:

- "Ever search the member list?  People leave strange files out there."  This is intended to prompt people to look at the SMWCentral
  member list.
- "People never pay much attention to who presents these hacks."  This is intended to lead people to reset the game and look at the
  "Nintendo Presents" screen, which says InalienableDreamless.

With these two pieces of information, search for InalienableDreamless on SMWCentral and look at their file bin.  There is a file 
named Beyond The Black Rainbow.  Inside is a short hack. 
After beating it, you can move down from The End to a level named Realization Time.

There are two info boxes:

- "bookhouse boys.  BeyonD the BLack RainBow BeyonD the BLack RainBow."   This is giving you the code to get past the first screen in Defiant Jazz.
  B, Down, B, Left, Right, B, B, Down, B, Left, Right, B.

- "Diane... Remember the time in my last note to you?  It is of profound significance."  There is a file named Diane.txt in the Beyond the Black
  Rainbow zip file.  Open that up and it mentions the time 4:28.  After you pass the bookhouse boys screen, you'll enter the How's Annie room.  Run 
  to number 4, then count 28 tiles and jump up to enter the invisible door.

After this, the puzzle is complete!



